The invention relates to the transmission of high speed digital data. Specifically, the invention provides a system and a method for decreasing the amount of crosstalk occurring within a data transmission system in which digital data is transmitted at high speeds over a telephone network.
The proliferation of home computers and the increase in usage of data networks, such as the Internet, has resulted in a marked increase in the amount of digital data transmitted over telephone networks. Data transmission systems utilizing voice band modems are approaching technological limits in efficiency and speed. Demand for both access to high speed data networks and higher data transmission rates has necessitated the development of new technologies for facilitating data transmission over traditional telephone lines.
One such newly developing family of technology involves digital subscriber lines (xDSL). For example, an Asymmetric Digital Subscriber Line (ADSL) allows for the transmission of both voice and data signals over the same twisted copper pair, at the same time. It allows a computer user access to high speed data networks, such as the Internet, at generally a faster rate than is available using traditional voice band modems. Since data signals are transmitted along with voice signals over traditional telephone lines, the ADSL user need not incur the expense of acquiring separate telephone lines or specialized data lines. Accordingly, demand for this technology has been considerable and its use within today's telephone networks is increasing rapidly.
In xDSL technologies, one end of the telephone line, usually of the twisted pair type, terminates at the location of the user in a subscriber unit. The other end of the telephone line terminates in a digital terminating unit at an access location within the telephone network, such as a central office or a remote access point such as a digital loop carrier. From this point, the user gains access to a digital data network, such as the Internet.
A problem arises when the telephone lines from multiple users transmitting data signals converge along the way to and at a common location such as the central office or digital loop carrier. Traditional telephone lines do not provide perfect separation between signals on different pairs. Some signal may leak from one pair and interfere with the signal on another nearby pair. This type of interference is known as crosstalk.
Two different types of crosstalk are known to those familiar in the art. These are Near-End Crosstalk (NEXT) in which the disturbed unit is located at the same end of the transmission line as the disturbing unit and Far-End Crosstalk (FEXT) in which the disturbed unit is located at the opposite end of the transmission line as the disturbing unit. In the case of xDSL systems as described herein, NEXT is alleviated by using a different frequency band for each direction of transmission and only FEXT remains as a possible reach limiting factor. Whenever mention is made of crosstalk in the present document, it is implied to be Far-End Crosstalk.
Traditional telephone lines provide much better separation and are less susceptible to crosstalk when they carry voice signals as opposed to data signals. While voice signals generally occupy bandwidth in the frequency range of 0 kHz to 4 kHz, data signals, in the case of xDSL technologies, occupy bandwidth on the telephone line at much higher frequencies; usually from 30 kHz and up to approximately the low megahertz range. For some xDSL systems, this range can extend to approximately 25 MHz. The physical characteristics of the traditional telephone lines are such that the higher data signal frequencies tend to leak more easily than the lower voice signal frequencies. Accordingly, the data signals carried over telephone lines are inherently susceptible to interference and crosstalk.
Exacerbating the problem is the disparity in the signal strengths of the data signals received at the central office. In existing xDSL systems, the strength of the data signal received at the central office depends, to a large degree, on the length and quality of the telephone line connecting the central office to the user's subscriber unit. Typically, the longer the distance between a user and the central office, the weaker the data signal received at the central office. Conversely, the data signal received from another user who is closer to the central office is much stronger.
When the telephone lines of two users converge along the way to and at the central office, especially if the lines are in physical proximity to each other, the stronger signal will tend to interfere and cause crosstalk with the weaker signal. The result is a decrease in the signal to noise ratio of the data signal from the more distant user and loss of data. As the number of such users in system increases and as more telephone lines carrying high speed data converge along the way to and at the central office, the potential for crosstalk and corruption of data increases exponentially.